1. Field of the Invention
The present invention relates to an erasable image forming material.
2. Description of the Related Art
Forest conservation is an essential requirement to maintain the terrestrial environment and suppress the greenhouse effect caused by CO2. In order to minimize additional tree trimming and to keep balance with forest regeneration including tree planting, it is important how to utilize the existing paper resources efficiently.
Currently, paper resources are “recycled” by recovering paper fibers from used paper through a deinking step of removing image forming materials printed on the used paper, remaking paper fibers to manufacture recycled paper with low paper quality, and using the recycled paper according to the purpose. Thus, problems of a high cost of the deinking step and possibility of new environmental pollution by waste fluid treatment are pointed out.
On the other hand, “reuse” of a hard copy has been put into practice through erasure of images, for example, by using an eraser for pencil images and a correcting fluid for ink images. Here, “reuse” in which a paper sheet is repeatedly used for the same purpose with preventing deterioration of paper quality as much as possible is different from “recycling” in which a paper sheet with deteriorated quality is used for other purposes. Now, the “reuse” can be said to be more important concept from a viewpoint of conservation of paper resources. If effective “reuse” at each “recycling” stage is performed, additional waste of paper resources can be minimized. Recently, for example, a rewritable paper has been proposed, which is a special paper intended to reuse hard copy paper. Use of the rewritable paper technology enables the paper to be “reused” 100 times or more if paper damage such as a wrinkle and fold due to use can be ignored, which greatly enhances the efficient use of paper resources.
However, the rewritable paper is a special paper which can be “reused” but cannot be “recycled”. The rewritable paper is also defective in that recording techniques other than thermal recording cannot be applied to.
The present inventors have paid their attention to a phenomenon caused by a system of a color former and a developer that a colored state is realized when interaction between the color former and the developer is increased and a decolorized state is realized when the interaction is decreased. Thus, the inventors have proposed, as paper reuse techniques, image forming materials of a composition system comprising a color former, a developer and a decolorizing agent. The image forming materials can exhibit stably a colored state around room temperature and can retain a decolorized state for a long time at practical temperatures by treatment with heat or a solvent. The inventors have also proposed image decolorizing processes and image decolorizing apparatuses for the image forming materials.
These image forming materials have advantages of high stability of colored and decolorized states of the images, highly safety in view of materials, applicability to electrophotography toners, liquid inks, ink ribbons and writing instruments, and feasibility of large-scale decolorizing treatment, which cannot be realized in any prior art.
The present inventors have further found that cellulose which is a constituent element of “paper” also has the decolorizing function, and proposed that even an image forming material not containing a decolorizing agent can be decolorized by treatment with heat or a solvent in applications of using paper for a recording medium.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-284520 discloses that, by using an image forming material containing a color former, a developer and a binder resin, a clear image can be formed and the image can be decolorized sufficiently. In this image forming material, the equilibrium between the color former and the developer is shifted to the non-coloring side when the material is heated, and the state shifted to the non-coloring side can be maintained by the binder resin when the material is cooled, so that the image can be decolorized.
Examples of the color former (known as a leuco dye) contained in the image forming materials include electron donating organic materials such as leucoauramines, diarylphthalides, polyarylcarbinols, acylauramines, arylauramines, rhodamine B lactams, azaphthalides, spiropyrans, and fluoranes.
Among leuco dyes, Crystal Violet Lactone (CVL) particularly shows an excellent heat decolorizing performance as compared with other leuco dyes. However, CVL has a problem that it is easily decomposed by light. Other leuco dyes may be excellent in light fastness, but do not show excellent heat decolorizing performance as compared with CVL.